1. Field of the Invention
The present invention pertains generally to optical communications receivers for data transfer to digital functions.
2. Description of the Background
Optical transmission of information has become increasingly popular with the advent of fiber optics, light emitting diodes (LED), and photo detectors. Typical communication paths have an origin, where an electrical signal is converted to an optical signal; a transmission path, where the optical signal travels, and a destination, where the optical signal is converted back into an intelligible electrical signal. At the origin of the transmission the information must be encoded into light. This can be done by current to light converters (typified by light emitting diodes). At the destination the light must be converted back into an electrical signal. In digital transmissions what is important is to detect the presence of light. Generally, the absence of light indicates a positive logic zero, while the presence of light indicates a positive logic one. When the destination input signal reaches a predetermined threshold, the amplifier must be able to detect the presence of the signal. The threshold is chosen to be a statistically significant amount of light greater than the noise level. When a transmitting signal generates light the amount of light showing up at the destination is a function of the strength of the electrical signal which generated the light, the efficiency of the transducer, the distance the light travelled from he generating site to the destination, the various coupling efficiencies, and the amount of scattering effects during the transmission. The electrical signal that is then generated is a function of the efficiency of the photo detector (light to current converter). Consequently, the destination input electrical signal for a positive logic one can vary greatly depending on the origin of the signal, length of the transmission, and the particular generators and detectors involved. An amplifier must be able to detect a positive logic one over a wide variety of destination input electrical signal levels. Also, in digital transmissions the electrical signal noise at the receiving end must be below the signal generated by the incoming light when the incoming light level signal is small. When the incoming light signal is large, the electrical signal must be able to respond fast enough to a light signal shutting off (transition to a positive logic zero). If the high level light signal is so strong that the electrical response stays high even after the light signal drops then the bandwidth of the signal will be restricted by the time it takes for the response to drop to the positive logic zero state and this limits the frequency and therefore speed of the digital transmission, slowing it down. Amplifying optical signals poses many problems with the present technology in optical communications. Since the intensity of the signal (light) varies according to the distance travelled through optical fibers, to the efficiency of the light generator, the coupling efficiencies, and to the efficiency of the light receiver, the same origin input electrical signal can thus generate a wide variety of light signal responses which must be able to be amplified by the receiving amplifier of the system. When producing a state-of-the-art amplifier in a mass production environment every amplifier produced must be able to respond to a wide range variation of input signals since it will not always be possible to match low efficiency generators with high efficiency light to current convertors (photodiodes) and vice versa. The amplifier must be able to operate in environments where low efficiency generators and convertors are combined giving rise to small input signals and where high efficiency generators and convertors are combined giving rise to large input signals. Also, even in a given combination the raw input signal to the light generator may sufficiently vary in strength as to produce a wide range of input signals for the amplifier.
Another consideration is the demand for increased speed of communications. Inverting amplifiers with noninverting feedback are limited to an odd number of inverting amplifiers connected in series. Inverting amplifiers have an output signal opposite to the input signal. For example, given an input current i, then the output voltage, v, will be a function of i, v=f(i). Often one inverting amplifier does not provide sufficient gain. While three inverting amplifiers provide enough gain for small signals, the delay time of the circuit may be too long. Two amplification stages would give the required gain, but inverting feedback would be necessary. Two inverted amplifiers connected in series would give a noninverted output signal. The output signal must, then, be inverted in the feedback in order to have negative feedback and a output commensurate with the input signal. Designing an amplifier with the requisite input dynamic range in prior technology results in adding too much delay in the system. For low signals adding an extra amplification stage results in too much delay. If delay is greater than one fourth of the period of the unity loop gain frequency the output signal will oscillate, and if the delay comes close to one fourth the period bad transient responses like ringing of the output signal occur. If the delay is decreased then the period of the unity loop gain frequency can be decreased which results in the increase of the unity loop gain frequency. As the unity loop gain frequency increases the receiver can handle a greater bandwidth. The unity loop gain frequency increases when the loop gain is increased. The loop gain is a product of the forward gain and feedback gain. In the prior art, amplification had to be an odd number of inverting amplification stages in order to produce an inverted output relative to the input so as to produce negative feedback with a noninverting feedback circuit. When an even number of inverting amplification stages would be sufficient, an extra inverting stage would be required so as to produce the necessary negative feedback. The extra stages add delay time to the system reducing stability. When the delays cause a 90 degree phase shift then the feedback becomes positive rather than negative. The loop gain must be decreased to handle more delay, thus maintaining negative feedback. This results in less bandwidth. Applicant's invention allows amplification to be in an even number of inverting stages at a time. This decreases the delay in the system. Because delay is decreased, loop gain can be increased resulting in a greater bandwidth receiver.
Present nonlinear inverting feedback has several problems: there are pattern dependent timing jitter problems, the transition from positive logic one to zero has an odd slow integrating decay, and at high input optical signal levels, it requires a wider bandwidth gain amplification circuit for stability.
Typically, either linear amplifiers or nonlinear inverting amplifiers with non-inverting feedback are used. Linear amplifiers have a limited dynamic input range because of the problem of saturation of the amplifier. A linear amplifier has an output signal linearly proportional to the input signal. For example, given an input signal i and an output signal v, then i=Gv where G is a transimpedance constant. If the linear amplifier is designed for the lowest level of input signal then the highest level input signal saturates the amplifier. The highest level input signal saturates the amplifier because of the large difference between the highest and lowest levels of input signals. Typically, the ratio can be 50:1. The problem is to maintain or increase the bandwidth of the amplifier while being able to handle the wide range of input signals from different LEDs. Linear inverting feedback would not provide the required dynamic range of the amplifier.
In order to make a linear amplifier work in this application it requires Automatic Gain Control (AGC) or limiting the bandwidth of the input signal which can be amplified. An AGC takes the output signal from the amplifier, processes it, and outputs a signal to the feedback circuit of the amplifier. Automatic Gain Control devices require switches or threshold detectors as well as amplifiers with a second feedback loop and a control element. This requires extra circuitry and thus, extra design effort and more space. There are concerns about stability, process fluctuations, reliability, manufacturing yield, and more potential for making more errors in the design, manufacture, and/or use of the amplifier. A receiver using an AGC requires substantially extra design effort of the receiver. An AGC must be designed to be stable at various control levels of voltage. An AGC includes a servo-loop and oscillating is a possibility. The second feedback loop incorporated into an AGC must be designed to be stable also. An AGC has restrictions on the type of signals it can receive, minimum data rates and minimum transitions per unit time must be specified. Although an AGC may provide a constant output signal response once the input has achieved a certain signal, the penalty of extra circuitry may not justify its use in applications where a constant response is not required or not required as rapidly as an AGC circuit may provide; the requirement is to avoid saturation of the amplifier within the expected input signal range and for the output signal to achieve a certain minimum level on a given input signal so a discriminator can recognize it as a positive logic one signal.